theyre really easy to change, pull the shaft out, pry out the old seal, then tap the new one in with an oversized socket or something. As far as purchasing one, I ran into the same problem, but Napa seems to always have them in stock, if not they can get them within a day or 2.

There is also a brass bushing in there that could be causing the leak, I had the seal replaced first but the leak is still there and I found out it is that bushing that is worn. Do not call the dealer about the bushing because they do not sell it, they will tell you that whole tailshaft housing needs to be replaced and it costs an arm and a leg. A transmission shop can press that bushing out and press it back in for a small fee. I still need to get it done on my 2000 2wd, I was quoted $150.